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Lithostratigraphic Units (Philippine Stratigraphic Guide)
Book source Philippine Stratigraphic Guide Chapter 4 A. NATURE OF LITHOSTRATIGRAPHIC UNITS Lithostratigraphic units are bodies of rocks, bedded or nonbedded, that are defined and characterized on the basis of their observable lithologic properties. Lithostratigraphic units are the basic units of geologic mapping and are an essential element of the stratigraphy of the area. A lithostratigraphic unit may consist of sedimentary, igneous or metamorphic rocks, or an association of two or more of these. Lithostratigraphic units are recognized and defined by observable physical features and not by their inferred age, the time span they represent, inferred geologic history, or manner of formation. Definition and recognition of lithostratigraphic units must be based on description of the lithologic composition of actual rock material, not on their geophysical (electrical, radioactive, density, sonic and other inferred or measured physical attributes) properties. Note 1. Boundaries in lateral lithologic change. Where a unit changes abruptly into, grades into, changes laterally through or intertongues with, a markedly different kind of rock, a new unit should be proposed for the different rock type. An arbitrary lateral boundary may be placed between the two equivalent units. Where the area of lateral intergradation or intertonguing is sufficiently extensive, a transitional interval of interbedded rocks may constitute a third independent unit. Where tongues of formations are mapped separately or otherwise set apart without being formally named, the unmodified formation name should not be repeated in a normal stratigraphic sequence, although the modified name may be repeated in such phrases as "lower tongue of ______" and "upper tongue of _____". To show the order of superposition on maps and cross sections, the unnamed tongues may be distinguished informally by number, letter or other means. Such relationships may also be dealt with informally through the recognition of depositional facies which may be descriptive (conglomeratic, black shale, graptolitic), or interpretative in genetic (turbiditic) or environmental (platform, fluvial) terms. B. KINDS OF LITHOSTRATIGRAPHIC UNITS 1. Formal Lithostratigraphic Units Formal lithostratigraphic units are those that are defined and named in accordance with an explicitly established or conventionally accepted scheme of classification and nomenclature. The conventional hierarchy of formal stratigraphic terms is as follows: Group – two or more formationsFormation – primary unit of lithostratigraphyMember – named lithologic entity within a formationBed – named distinctive layer in a member or formationFlow – smallest distinctive layer in a volcanic sequence 2. Formation The formation is the fundamental unit in lithostratigraphic classification. It is a body of rock identified by lithologic characteristics and stratigraphic position, usually but not always tabular and mappable at the earth’s surface or traceable in the subsurface. Note 1. Fundamental unit. Formations are the basic lithostratigraphic units used in describing and interpreting the geology of a region. The limits of a formation normally are those surfaces of lithologic change that give it the greatest practicable unity of constitution. A formation may represent a long or a short time interval, may be composed of materials from one or several sources, and may include breaks in deposition. Note 2. Content. A formation should possess a certain degree of internal lithologic homogeneity or distinctive lithologic features. It may contain between its upper and lower limits (I) rock of one lithologic type; (ii) repetitions of two or more lithologic types; or (iii) extreme lithologic heterogeneity which in itself may constitute a form of unity when compared to the adjacent rock units. Note 3. Lithologic characteristics. Distinctive lithologic characteristics include chemical and mineralogical composition, texture, and such supplementary features as color, primary sedimentary structures or volcanic depositional structures, fossils (viewed as rock-forming particles), or other organic content (coal, oil-shale). A unit distinguishable only by the taxonomy of its fossils is not a lithostratigraphic but a biostratigraphic unit. Rock type may be distinctively represented by electrical, radioactive, seismic or other properties, but these properties by themselves do not describe adequately the lithologic character of the unit. Note 4. Mappability and thickness. A unit to be proposed for a new formation must have been tested for its mappability. Formations may be divisible into several recognizable sub/smaller lithostratigraphic units. Where formal recognition of these smaller units serves a useful purpose, they may be established as members and beds; the requirement of mappability for members and beds is not mandatory. Thickness is not a determining parameter in dividing a rock succession into formations. The thickness of a formation may range from less than a meter to several thousands of meters. A proposed formation is considered invalid if it cannot be delineated at the scale of geologic mapping, preferably at a scale of 1:25,000. A formation may be represented on maps and cross sections by a labeled line; however, too many thin lines representing too many thin units are not really recommended. Subsurface mapping methods permit delineation of units much thinner than those usually practicable for surface studies; however before such thin units are formalized, their effect on subsequent surface and subsurface studies should be considered. Note 5. Organic reefs and carbonate mounds. Organic reefs and carbonate mounds may be distinguished formally, if desirable, as formations and distinct from their surrounding thinner, temporal equivalents. For the requirements of formalization, see Chapter 2, Section B. Note 6. Interbedded volcanic and sedimentary rocks. Sedimentary and volcanic rocks that are interbedded may be assembled into a formation under one name, which should indicate the predominant or distinguishing lithology, such as Pulangi Volcanics. Note 7. Volcanic rocks. Mappable, distinguishable sequences of stratified volcanic rocks should be treated as lithostratigraphic units, as formations or as units of higher or lower rank. A small intrusive component of a dominantly stratiform volcanic assemblage may be treated informally. Note 8. Metamorphic rocks. Formations composed of low-grade metamorphic rocks (defined for this purpose as rocks in which primary structures are clearly recognizable) are, like sedimentary formations, distinguished mainly by lithologic characteristics. The mineral facies may differ from place to place, but these variations do not require definition of a new formation. However, high-grade metamorphic rocks, whose relationships to established formations are uncertain, are treated as Special Lithostratigraphic Units. 3. Member A member is the formal lithostratigraphic unit next in rank below a formation, and is always a part of the same formation. It is recognized as a named entity within a formation because it possesses characteristics distinguishing it from adjacent parts of the formation. A formation need not be divided into members unless a useful purpose is served by doing so. Some formations may be divided completely into members; others may have only certain parts designated as members; still others may have no members. Note 1. Mapping of members. A member is established when it is advantageous to recognize a particular part of a heterogeneous formation. A member, whether formally or informally designated, need not be mappable at the scale required for formations. Even if all members of a formation are locally mappable, they should not be raised to formational rank, if they obscure rather than clarify relations with other areas. A member may extend laterally into an adjacent formation(s). Note 2. Lens and tongue. A geographically restricted unit that terminates on all sides laterally within a formation may be called a lens. A wedging unit may be called a tongue. Note 3. Division of members. A formally or informally recognized division of a member is called a bed or beds. For volcanic-flow rocks, the smallest formal unit is a flow. Members may contain beds or flows, but may never contain other members. Note 4. Laterally equivalent members. Although members are normally in vertical sequence, laterally equivalent parts of a formation that differ recognizably may also be considered members. 4. Bed A bed is the smallest formal lithostratigraphic unit of sedimentary rocks. The designation of a bed or a unit of beds as a formally named lithostratigraphic unit generally should be limited to certain distinctive beds whose recognition is particularly useful. Several contiguous beds of similar lithology may also together constitute a formal unit. Coal beds, oil sands, and other beds of economic importance commonly are named. Such units and their names usually are not a part of formal stratigraphic nomenclature, for example, "Camp 3 Beds" designated by Mitchell and Leach (1991) for the beds that are part of the Zigzag Formation at Camp 3, Tuba, Benguet, underlying the Kennon Limestone. Note 1. Key or marker beds. A key or marker bed is a thin bed of distinctive rock that is widely distributed. Such beds may be named, but usually are considered informal units. Individual key beds may be traced beyond the lateral limits of a particular formal unit. 5. Flow A flow is the smallest formal lithostratigraphic unit of volcanic flow rocks. A flow is a discrete, extrusive, volcanic body distinguishable by texture, composition, order of superposition, paleomagnetism, or other objective criteria. It is equivalent in rank to a bed in sedimentary rock classification. Many flows are informal units. For example, Listanco (1998) defined the various deposits that make up the Taal Caldera, designating the flow units as Alitagtag Pumice Flow, Caloocan Pumice Flow, Sambong Ignimbrite and Taal Scoria Flow. The designation and naming of flows as formal rock-stratigraphic units should be limited to those that are distinctive and widespread. 6. Group A group is the lithostratigraphic unit next higher in rank formation. Groups are defined to express the natural relationships of associated formations. They are useful in regional mapping and stratigraphic analysis. In some reconnaissance work, the term "group" may be applied to lithostratigraphic units that appear to be divisible into formations, but have not yet been so divided. In such cases, formations may be erected subsequently for one or all of the practical divisions of the group. An example is the Caraballo Group in northeastern Sierra Madre of Luzon, consisting of volcano-sedimentary formations designated as Formations I, II and III (MMAJ, 1977). Note 1. Change in component formations. The formations making up a group need not necessarily be everywhere the same. However, a formation or its parts may not be assigned to two vertically adjacent groups. Note 2. Change in rank. The wedging-out of a component formation or formations does not justify the reduction of a group to formation rank while retaining the same name (e.g. X______Group to X_______ Formation). When a group is extended laterally beyond where it is divided into formations, it becomes in effect a formation, even if it is still called a group. When a previously established formation is divided into two or more component units that are given formal formation rank, the old formation, with its old geographic name, should be raised to group status. Raising the rank of the unit is preferable to restricting the old name to a part of its former content, because a change in rank leaves the sense of a well-established unit unchanged. 7. Supergroup A supergroup is a formal assemblage of related or superposed groups, or of groups and formations. Such units have proved useful in regional and provincial syntheses. Supergroups should be named only where their recognition serves a clear purpose. Note. Misuse of "series" for group or supergroup. Although "series" is a useful general term, it is applied formally only to a chronostratigraphic unit. The term "series" (e.g Ilocano series of Hashimoto (1939) consisting of the Malaya Formation and Ata volcanic rocks) should no longer be employed for an assemblage of formations and groups as previously used. C. LITHOSTRATIGRAPHIC NOMENCLATURE The formal name of a lithostratigraphic unit is compound and binary. It consists of a geographic name combined with a descriptive dominant lithologic term or the appropriate rank term. The initial letters of all words used in forming the names of formal rock-stratigraphic units are capitalized. 1. Use of Simple Lithologic Terms The lithologic part of the name should indicate the predominant or diagnostic lithology, even if subordinate lithologies are included. Where a lithologic term is used in the name of a lithostratigraphic unit, the simplest generally acceptable term is recommended (e.g. limestone, sandstone, shale, tuff, quartzite). Compound terms (e.g. carbonaceous shale) and terms that are not in common usage (calcirudite, orthoquartzite) should be avoided. Combined terms, such as "sand and clay" should not be used for the lithologic part of the names of lithostratigraphic units (for example, Malabago conglomerate and shale of Corby and others, 1951), nor should an adjective be used between the geographic and lithologic terms (e.g. Cebu Orbitoid Limestone of Corby and others, 1951). 2. Naming of Rock-stratigraphic Units *''Group name'' – A group name combines a geographic name with the term "group" and no lithologic designation is included (e.g. Bued River Group). *''Formation name'' – a formation name consists of a geographic name followed by a lithologic designation or by the word "Formation", e.g. Klondyke Formation of the Bued River Group. *''Member name'' – A member name includes a geographic term and the word "member". Some have an intervening lithologic designation, if useful (Pau Sandstone Member of the Malinta Formation in Tarlac). Members designated solely by lithologic character (e.g. limestone member), by position (upper, lower) or by letter or number are informal. *Bed and flow names – The name of beds or flows may combine a geographic term, lithologic term (optional) and the term "Bed" or "Flow". 3. Informal Units. When geographic names are applied to such informal units as oil sands, coal beds, mineralized zones, and informal members, the unit term should not be capitalized. D. Special Lithostratigraphic Units Intrusive igneous and other non-layered rocks characterized by absence of stratification or layering that define the succession of rocks in terms of superposition may be regarded as special units with unique aspects. These rocks may be defined, classified and mapped on the basis of their lithologic characteristics and stratigraphic relationships as special categories of lithostratigraphic units. 1. Stratigraphic Relationships Although in many cases the precise stratigraphic positions of these units with respect to stratified units are not clearly defined in terms of the Law of Superposition, they can contribute to a clear determination of stratigraphic succession with reference to their geochronometric ages as determined by isotopic or other methods and provide stratigraphic information through the establishment of their cross-cutting and boundary relationships with adjacent layered and/or non-layered rocks. 2. Naming The names of bodies of intrusive igneous rocks and other non-layered rocks of undetermined origin should be composed of an appropriate local geographic term combined with either a unit-term indicating rank or a simple field lithologic term indicating the dominant rock type. All terms of the name should be capitalized, e.g. Virac Granodiorite. However, since the terms "group," "formation" and "member" generally imply stratified units, it may be more appropriate to use simple field lithologic terms such as "diorite" or "schist". The same rules for the naming of sedimentary rocks are applied to non-layered rocks. The proposal of a new unit or the redefinition of an already established one should include a comprehensive and clear description of the unit, express the intent to introduce a new unit or to revise an existing one, designate a type locality (and reference localities, if appropriate) cite the geographic feature from which the name of the unit is taken, and so on. Note 1. Use of adjectival qualifiers, The use of adjectival qualifiers such as "plutonic," igneous," "intrusive," "extrusive" and such like should be minimized in the formal nomenclature of lithostratigraphic units. They may be used, however, when they help clarify the nature of a unit or provide valuable descriptive information. Note 2. Adjectives used as nouns. '''Adjectives used as nouns, such as "volcanics," "metamorphics," "intrusives," and "extrusives," preferably should not be used in naming igneous and metamorphic rock bodies. However, they may be reluctantly accepted in formal lithostratigraphic nomenclature when judged useful. For example, the term "Suyo Metamorphics" named for the schists in Ilocos Norte could be changed to "Suyo Schist" which is a more acceptable appelation. ''Note 3. Terms that express form or structure. '''Lithostratigraphic names of igneous and metamorphic rocks should not include terms that express form or structure such as "dike," "batholith," "pluton" "diapir," "stock," "pipe" and "neck" or the more general term "intrusion." These terms do not indicate the lithology of the rock body, are not unit terms in the lithostratigraphic hierarchy, and are therefore, not lithostratigraphic terms. A few examples of such misuse include, "Agno Batholith" and "Balatoc Plug". 3. Use of "Complex" The term "complex" may be used for igneous and/or metamorphic rock bodies of diverse and irregularly mixed lithology, whether or not they are strongly deformed and/or metamorphosed. ''Note 1. Volcanic complex. "Volcanic complex" may be used to describe a diverse assemblage produced by volcanic activity to include extrusive volcanic rocks, related intrusions and associated pyroclastic deposits. An example is the Pliocene-Pleistocene dacites and andesites, breccias and associated tuffs around the Lepanto mine in Mankayan, Benguet, which may altogether be called Mankayan volcanic complex (not Lepanto volcanic complex because there is a unit previously named as "Lepanto Metavolcanics"). Note 2. Use of "suite." The term "suite" has been commonly used for associations of apparently co-magmatic intrusive igneous rock bodies of similar or related rock lithologies and close association in time, space, and origin. Thus we may treat the Virac Granodiorite as the Granodiorite Suite of the Central Cordillera Diorite Complex. We may also regard layered gabbro as a suite of an ophiolite body. 4. Ophiolites Ophiolites are assemblages of rock bodies with distinct lithologic characteristics and genetic associations. In a sense, they are a special class of complexes. A complete ophiolite sequence consists of the following, from the bottom to the top: amphibolite, residual peridotite (harzburgite), transition zone dunite, layered ultramafic cumulates, layered gabbro, isotopic high level gabbro, sheeted dikes/sills complex, pillow basalt and pelagic sedimentary rocks. Each mappable rock body with distinct lithology may be regarded as a distinct unit. Previously, mapping work in the Philippines defined the sedimentary carapace and the pillow basalts as separate formations without associating them with the ultramafic complexes nearby. Later, they were recognized as components of ophiolite. For practical considerations, the sedimentary carapace may be defined as a separate formation, without ignoring the fact that they are part of an ophiolite or ophiolitic complex, in much the same way that a formation could be part of a group. Note 1. Incomplete ophiolite. In case the assemblage does not represent a complete ophiolite sequence, the composite body may be called an ophiolitic complex. 5. Melange A melange is a kind of complex consisting of a variety of rocks in a fine-grained matrix. Although melanges may originate in a number of ways, e.g. tectonic, igneous, sedimentary, the definition to be adopted here is descriptive, as defined by Raymond (1984). Thus a melange is here defined as a body of mappable rock unit (1:50,000) characterized both by the lack of internal continuity of contacts or strata and by the inclusion of fragments and blocks of all sizes, both exotic and native, in a matrix of finer-grained material. Exotic block denotes a fragment of rock greater than 64 mm in diameter that was not interbedded or intimately associated with the dominant formation that give rise to the matrix. Examples include Baruyen Formation in Ilocos Norte and Paniciuan Melange in Antique. 6. Olistostrome. Olistostrome is defined here as a mappable sedimentary slide deposit characterized by lithologically heterogenous bodies of harder rock mixed and dispersed in a matrix of prevalently pelitic heterogeneous material. It is distinguished from melange by the absence of exotic blocks. An example is the "Codon Olistostome" in Catanduanes.